Part cast made from aluminum alloy with high hot strength

ABSTRACT

Cast part with high creep resistance, made of an alloy with a composition comprising (% by weight): Si: 5-11 Fe&lt;0.6 Mg: 0.15-0.6 Cu: 0.3-1.5 Ti: 0.05-0.25 Zr: 0.05-0.25 Mn&lt;0.4 Zn&lt;0.3 Ni&lt;0.4 other elements&lt;0.10 each and 0.30 total, remainder aluminium.

DOMAIN OF THE INVENTION

The invention relates to cast parts made of aluminium alloy subjected tohigh thermal and mechanical stresses, particularly cylinder heads andcrankcases of internal combustion engines, and more particularlyturbocharged diesel or gasoline engines. It also relates to parts otherthan automobile parts subjected to the same types of stress, for examplein mechanics or in aeronautics.

STATE OF THE ART

Two families of aluminium alloys are usually used for the manufacture ofengine cylinder heads:

1) alloys containing 5 to 9% of silicon, 3 to 4% of copper andmagnesium. They are usually secondary alloys, with iron contents ofbetween 0.5 and 1%, and fairly high contents of impurities, particularlymanganese, zinc, lead, tin or nickel. These alloys are generallynon-heated alloys (temper F) or simply stabilised (temper T5). They areused particularly for manufacturing cylinder heads for gasoline engineswith fairly low temperature stresses. For more highly stressed partsintended for diesel or turbo-diesel engines, primary alloys are usedwith an iron content of less than 0.3%, heat treated to temper T6(tempered to the peak mechanical strength) or T7 (over-ageing).

2) primary alloys containing 7 to 10% of silicon and magnesium treatedto temper T6 or T7, for more highly stressed parts like those intendedfor use in turbo-diesel engines.

These two large families of alloys require different compromises betweenvarious usage properties: mechanical strength, ductility, creepresistance and fatigue resistance. For example, this problem has beendescribed in the article by R. Chuimert and M. Garat: “Choice ofaluminium casting alloys for highly stressed diesel cylinder heads”,published in the SIA Review, March 1990. This article summarises theproperties of 3 alloys studied as follows:

Al—Si5Cu3MgFe0.15 T7: high strength—good ductility

Al—Si5Cu3MgFe0.7 F: high strength, low ductility

Al—Si7Mg0.3Fe0.15 T6: low strength, extreme ductility.

The first and third alloy-temper combinations may be used for highlystressed cylinder heads. However, attempt continued to find an improvedcompromise between strength and ductility. Patent FR 2690927 issued bythe inventors and deposited in 1992 describes aluminium alloys resistantto creep containing 4 to 23% of silicon, at least one element amongmagnesium (0.1-1%), copper (0.3-4.5%) and nickel (0.2-3%), and 0.1 to0.2% of titanium, 0.1 to 0.2% of zirconium and 0.2 to 0.4% of vanadium.An improved creep resistance is observed at 300° C. with no significantloss in the elongation measured at 250° C.

The article by F. J. Feikus “Optimisation of Al—Si cast alloys forcylinder head applications” AFS Transactions 98-61, pp. 225-231, studiesthe addition of 0.5% and 1% of copper to an AlSiMg0.3 alloy formanufacturing cylinder heads for internal combustion engines. Noimprovement in the yield stress and no increase in the hardness atambient temperature was observed after conventional T6 treatmentinvolving 5 h dissolution at 525° C. followed by quenching in cold waterand annealing for 4 h at 165° C. The added copper only makes asignificant improvement to the yield stress and creep resistance atusage temperatures of more than 150° C.

Patent EP 1057900 (VAW Aluminium) deposited in 1999 is a developmentalong the same direction and describes the addition of closelycontrolled quantities of iron (0.35-0.45%), manganese (0.25-0.30%),nickel (0.45-0.55%), zinc (0.10-0.15) and titanium (0.11-0.15%) to anAl—Si7Mg0.3Cu3.5 alloy. This alloy has good creep resistance, highthermal conductivity, satisfactorily ductility and good resistance tocorrosion, in the T6 and T7 tempers.

The purpose of this invention is to further improve the mechanicalstrength and creep resistance of cast parts made of AlSiCuMg type alloyswithin the temperature range 250-300° C., without degrading theirductility, and avoiding the increased use of alloying elements that cancause problems in recycling.

PURPOSE OF THE INVENTION

The purpose of the invention is a cast part with high mechanicalstrength when hot and high creep resistance, made of an alloy withcomposition (% by weight): Si: 5-11 and preferably 6.5-7.5 Fe <0.6 andpreferably <0.3 Mg: 0.15-0.6 ″ 0.25-0.5 Cu: 0.3-1.5 ″ 0.4-0.7 Ti:0.05-0.25 ″ 0.08-0.20 Zr: 0.05-0.25 ″ 0.12-0.18 Mn <0.4 ″ 0.1-0.3 Zn<0.3 ″ <0.1 Ni <0.4 ″ <0.1

other elements<0.10 each and 0.30 total, remainder aluminium.

The part is preferably solution heat treated, quenched and tempered toT6 or T7.

DESCRIPTION OF THE INVENTION

The invention is based on the observation made by the inventors that ifa small quantity of zirconium is added to a silicon alloy containingless than 1.5% of copper and less than 0.6% of magnesium, it is possibleto obtain good mechanical strength and good creep resistance within the250-300° C. range of cast parts tempered to T6 or T7 with no loss ofductility. This result is obtained without needing to use elements suchas nickel or vanadium that cause problems in recycling. Furthermore,nickel has the disadvantage that it reduces the ductility of the part.

Like most alloys intended for the manufacture of engine cylinder heads,the alloy contains 5 to 11% of silicon and preferably 6.5 to 7.5%. Ironis kept below 0.6% and preferably below 0.3%, which means that primaryor secondary alloys can be used, preferably primary alloys when a highelongation at failure is required.

Magnesium is a normal alloying element for alloys used in cylinderheads; if its content is equal to at least 0.15% and in combination withcopper, it improves mechanical properties at 20 and 250° C. Beyond 0.6%,there is a risk of reducing the ductility at ambient temperature.

The addition of 0.3 to 1.5% and preferably 0.4 to 0.7% of copper canimprove the mechanical strength without affecting the corrosionresistance. The inventors have also observed that within these limits,the ductility and strength of parts in the T6 or T7 temper when hot arenot reduced. It also surprisingly transpired that the mechanicalstrength when hot and the creep resistance at 250° C. are significantlyimproved if the contents of Cu and Mg in % increase jointly within thelimits given above respecting the condition 0.3Cu+0.18<Mg<0.6.

At a content of more than 0.1%, manganese also has a positive effect onthe mechanical strength at 250° C., but this effect reaches the maximumabove a content of 0.4%. The titanium content is kept between 0.05 and0.25%, which is fairly normal for this type of alloy. Titaniumcontributes to refining the primary grain during solidification, but inthe case of alloys according to the invention, it works in liaison withzirconium and also contributes to the formation of very fine dispersoids(<1 μm) of AlSiZrTi in the in-body part of the α-Al solid solutionduring the solution heat treatment of the cast part, these dispersoidsbeing stable above 300° C., unlike the Al₂CuMg, AlCuMgSi, Mg₂Si andAl₂Cu phases that coalesce starting from 150° C.

These dispersoid phases do not cause embrittlement, unlike the largeAlSiFe and AlSiMnFe iron phases (20 to 100 μm), and nickel phases thatare formed during casting into interdendritic spaces.

Parts are made by normal casting processes, particularly chill castingby gravity and low pressure casting for cylinder heads, but also sandcasting, squeeze casting (particularly in the case of insertion ofcomposites) and lost foam casting.

Heat treatment comprises solution heat treatment typically for 3 to 10 hat a temperature of between 500 and 545° C., quenching preferably incold water, waiting between quenching and annealing for 4 to 16 h andannealing from 4 to 10 h at a temperature between 150 and 240° C. Theannealing temperature and duration are adjusted so as to obtain eitherannealing to the peak mechanical strength (T6) or over-ageing (T7).

Parts according to the invention, and particularly cylinder heads andcrankcases of automobile or aircraft engines, have a high mechanicalstrength, good ductility, and higher mechanical strength when hot andcreep resistance than parts according to prior art.

EXAMPLES Example 1

100 kg of an alloy A with the following composition (% by weight) wasproduced in the silicon carbide crucible in an electric furnace:

Si=7.10 Fe=0.15 Mg=0.37 Ti=0.14 Sr=170 ppm

100 kg of alloy B with the same composition with an added 0.49% coppercontent

100 kg of alloy C with the same composition as B with an added 0.14%zirconium content.

These compositions were measured by spark emission spectrometry, exceptfor Cu and Zr that were measured by induced plasma emissionspectrometry.

Fifty AFNOR tensile chill test pieces were cast for each alloy. Thesetest pieces were subjected to a heat treatment comprising solution heattreatment for 1 h at 540° C., preceded by a constant period of 4 h at500° C. for the copper alloys B and C to prevent burning, quenching incold water, natural ageing at ambient temperature for 24 h and annealingfor 5 h at 200° C.

Tensile test pieces and creep test pieces were machined from these testpieces so as to measure mechanical properties (ultimate strength R_(m)in MPa, yield stress R_(p0.2) in MPa and elongation at failure A in %)at ambient temperature, at 250° C. and at 300° C. The results areindicated in table 1: TABLE 1 R_(m) R_(p0.2) A R_(m) R_(p0.2) A R_(m)R_(p0.2) A Temp. Amb. Amb. Amb. 250° C. 250° C. 250° C. 300° C. 300° C.300° C. A 299 257 9.9 61 55 34.5 43 40 34.5 B 327 275 9.8 73 66 34.5 4440 34.6 C 324 270 9.8 68 63 34.5 45 42 35.0

It is found that the addition of copper to alloy A improves themechanical strength without modifying the elongation, both cold and hot,and that the addition of zirconium to B has almost no influence on themechanical properties.

The next step was to measure the elongation (in %) after 100 h at 250°C. and 300° C. at different stresses (in MPa), on the creep test piecesfor alloys B and C. Table 2 shows the result: TABLE 2 Temperature (° C.)250 250 300 Stress (MPa) 45 40 22 A (%) alloy B 2.43 0.134 0.136 A (%)alloy C 0.609 0.079 0.084

It is found that for identical temperature and stress, alloy C with theadded zirconium has a significantly better creep resistance, thedeformation under constant load being reduced by 40 to 75% depending onthe case.

Example 2

10 test pieces of each of the five alloys D to H were prepared under thesame conditions as for alloy C, in example 1, varying the copper contentand magnesium content within the preferred composition limits mentionedabove. The compositions of the alloys are given in table 3: TABLE 3Alloy Si Cu Mg Zr Ti D 7.1 0.4 0.3 0.14 0.12 E 7.1 0.4 0.4 0.14 0.12 F7.1 0.5 0.35 0.14 0.12 G 7.1 0.65 0.3 0.14 0.12 H 7.1 0.65 0.4 0.14 0.12

The mechanical properties at 20° C. and 250° C. were measured in thesame manner. The results corresponding to the average of the valuesobtained on the test pieces of each alloy are given in table 4: TABLE 4R_(m) (MPa) R_(0.2) (MPa) A (%) R_(m) (MPa) R_(0.2) (MPa) A (%) Alloy20° C. 20° C. 20° C. 250° C. 250° C. 250° C. D 301 250 8.9 69 60 44.5 E325 282 7.6 77 66 36.3 F 320 271 8.7 74 63 41.5 G 315 259 9.1 71 60 45.2H 339 291 8.7 81 69 39.6

Within the tested composition limits, it is found that the ultimatestrength and the yield stress increase as the Cu and Mg contentsincrease, but that elongation is not very much affected. At 250° C., theincrease in the Mg content from 0.3 to 0.4% has a very good effect onthe ultimate strength and the yield stress, particularly for the alloywith the highest copper content (H).

Furthermore, for an equal copper content, the increase of the magnesiumcontent from 0.3 to 0.4% improves the creep resistance at 250° C., ascan be seen from the results of the creep tests at a stress of 40 MPaafter 100, 200 and 300 h for alloys G and H, as indicated in table 5:TABLE 5 Duration 100 h 200 h 300 h ε (%) G 0.098 0.48 1.20 ε (%) H 0.0780.18 0.31

Example 3

Test pieces of 6 alloys I to N with the compositions indicated in table6 were prepared in the same way as for alloy C in example 1: TABLE 6Alloy Si Cu Mg Mn Zr Ti I 7 0.5 0.3 — 0.14 0.12 J 7 0.5 0.3 0.15 0.140.12 K 7 1 0.3 — 0.14 0.12 L 7 1 0.3 0.15 0.14 0.12 M 7 1 0.3 0.25 0.140.12 N 7 1 0.5 0.25 0.14 0.12

Mechanical properties were measured at 250° C. and table 7 shows theresults: TABLE 7 Alloy R_(m) (MPa) R_(0.2) (MPa) A (%) I 73 62 45 J 7665 37 K 70 59 46 L 77 62 47 M 77 62 46 N 90 75 33

It can be found that the addition of 0.1 to 0.3% of manganese increasesthe mechanical strength at 250° C. by at least 5%. However, there is noincrease between 0.15 and 0.25%. Finally, for an alloy N with highcopper content, the increase in the magnesium content from 0.3 to 0.5%gives a spectacular and unexplained increase in the mechanical strengthwhen hot.

1. Cast part with high creep resistance, made of an alloy with a composition comprising (% by weight): Si: 5-11 Fe<0.6 Mg: 0.15-0.6 Cu: 0.3-1.5 Ti: 0.05-0.25 Zr: 0.05-0.25 Mn<0.4 Zn<0.3 Ni<0.4 other elements<0.10 each and 0.30 total, remainder aluminium.
 2. Part according to claim 1, wherein silicon is from 6.5 to 7.5%.
 3. Part according to claim 1, wherein iron is at most 0.3%.
 4. Part according to claim 1, wherein copper is from 0.4 to 0.7%.
 5. Part according to claim 1, wherein magnesium is from 0.25 to 0.5%.
 6. Part according to claim 1, wherein magnesium and copper are present such that 0.3Cu+0.18<Mg<0.6.
 7. Part according to claim 1, wherein titanium is from 0.08 to 0.20%.
 8. Part according to claim 1, wherein zirconium is from 0.12 to 0.18%.
 9. Part according to claim 1, wherein manganese is from 0.1 to 0.3%.
 10. Part according to claim 1, wherein zinc is at most 0.1%.
 11. Part according to claim 1, wherein nickel is at most 0.1%.
 12. Part according to claim 1, wherein said part is solution heat treated, quenched and tempered to T6 or T7.
 13. Part according to claim 1, wherein said part is a cylinder head or a crankcase of an automobile or aircraft engine.
 14. Part according to claim 2, wherein iron is at most 0.3%.
 15. Part according to claim 2, wherein copper is from 0.4 to 0.7%.
 16. Part according to claim 3, wherein copper is from 0.4 to 0.7%.
 17. Part according to claim 2, wherein magnesium is from 0.25 to 0.5%.
 18. Part according to claim 3, wherein magnesium is from 0.25 to 0.5%.
 19. Part according to claim 4, wherein magnesium is from 0.25 to 0.5%.
 20. Part according to claim 2, wherein magnesium and copper are present such that 0.3Cu+0.18<Mg<0.6. 